This invention relates to carburetors for internal combustion engines, and more particularly to devices which enable the richness and throughput of the mixture which feeds motor vehicle engines to be increased.
In carburetors of a known type, a throttle valve controls the mixture flow rate to be fed to the engine. In order to enrich the mixture during cold-engine starting and operation, a choke valve disposed upstream of the choke portion is urged into its closed position by a force which progressively diminishes as the engine temperature increases, starting from a relatively high value to a negligible value which enables the choke valve to be completely opened when thermal equilibrium is reached.
As the mixture richness increases, its flow rate must also suitably increases. For this purpose, it is known, during the heating-up stage, to increase the small opening of the throttle valve, and the throttle valve is progressively returned to its minimum opening position as the engine temperature rises.
Means and systems are known which operate in such a manner as to associate the value of the small opening of the throttle valve with the intensity of the closure force on the choke valve. It is also known to use temperature-sensitive elements subjected to the heating action of a fluid which represents the engine temperature, these elements containing a plastic material of low melting point, the expansion of which enables both the choke valve mechanism and the mechanism which determines the small opening of the throttle valve to be positioned.
These temperature-sensitive elements are able to completely automate the starting devices, i.e. they are automatically put into operation when the engine cools, and are de-activated when the engine heats up.
In known arrangements, the temperature-sensitive element acts on a lever pivoted on the throttle valve spindle.
As the temperature varies, the temperature-sensitive element varies the position of the lever, which in its turn varies the intensity of the closure force on the choke valve and the position of the small opening of the throttle valve.
An arrangement of this type has particular drawbacks, such as not being able to guarantee a reliably valid relationship between the intensity of the closure force on the choke valve and the small opening of the throttle valve, because of the cumulative dimensional tolerance of the members which constitute the device, this cumulative tolerance being inevitable in industrial production. Moreover, by entrusting the positioning of the small opening of the throttle valve to the lever pivoted on the throttle valve spindle, the device becomes especially subject to the dangers deriving from vibration induced by the engine.
In present-day carburetion techniques, it is required to differentiate between the temperature of complete opening of the choke valve and the temperature at which a small opening of the throttle valve greater than minimum opening has to be still maintained. This is in order to reduce excessive pollution on the one hand, and to prevent the risk of engine stoppage on the other.
The known devices proposed for this purpose use complicated configurations of members, the definition, construction, assembly and adjustment of which are particularly difficult and costly.